Tetrafluoroethylene resin (hereinafter abbreviated as PTFE) is utilized, owing to excellent properties thereof such as a chemical resistance and a heat resistance, in engineering plastics, electronic parts, medical materials and the like. In particular, a porous article utilizing PTFE as the base material (PTFE porous member) is utilized in filters such as a gas separating membrane and a liquid separating membrane utilizing the uniform and fine porous structure thereof, and in medical materials such as an artificial blood vessel and in a degassing or gas-dissolving membrane for an aqueous liquid, utilizing a water-repellent property thereof.
The PTFE porous article, employed in a filter or the like, is required to have a fine pore size for enabling a filtered separation of fine particles, a permeability and a permeating flow rate for obtaining a satisfactory filtration efficiency, and a satisfactory strength. Also a high porosity is desired in order to obtain a satisfactory permeability and a satisfactory permeating flow rate.
As a producing method for such PTFE porous article, JP-B-42-13560 (Patent Reference 1) describes a method of mixing a liquid lubricant (additive) with PTFE fine powder as a raw material to form a press-molded substance, then extruding it into a predetermined form, and expanding the extruded article along the axial direction to form a porous structure, followed by a sintering. In addition to such expanding method, other methods of forming fine pores are known such as a method of mixing a pore-forming agent and later extracting the pore-forming agent, and a foaming method utilizing a foaming agent.
In these commonly utilized methods, the press-molded substance of PTFE particles contains voids resulting from gaps between the particles and from elimination of the additive, and such voids constitute a limit in forming small pores. It is therefore difficult, by these methods, to obtain a filter enabling removal of particles of a size less than 0.1 μm.
Consequently proposed is a method of once fusing the press-molded substance thereby causing a contraction and extinguishing the gaps between the particles and then forming fine pores by an expanding, and for example JP-A-53-55379 (patent reference 2) describes a method of heating a molded substance of PTFE particles to 327° C., which is the melting temperature of PTFE, or higher, and executing a gradual cooling after heating to promote a crystallization, followed by an expanding. This method allows to extinguish the gaps of the particles in the PTFE molded substance by fusing, and, by the subsequent expanding, to form fine pores of 0.1 to 100 μm or even smaller. It is however difficult, with this method, to obtain a high porosity of 30% or higher (corresponding to a density of 1.58 g/cc or less) while maintaining the fine pores.
The porosity is considered to be improved by increasing the expand rate. Also JP-A-61-146522 (patent reference 3) describes that, even in a molded substance that is already sintered, the expand rate thereof can be increased by an irradiation with an ionizing radiation of a dose of from 10 Gy to 10 kGy (from 103 to 106 rad).
However, in the prior expanding method of PTFE, an increase in the expand rate increases the porosity but also causes an increase in the pore size. Therefore, in case of obtaining a filter of a small pore size enabling removal of fine particles of a size less than 0.1 μm, as necessitated in the highly advanced integration level of semiconductors, the expand rate cannot be made high, so that the porosity becomes lower to result in an extremely low filtering ability.
Patent reference 1: JP-B-42-13560
Patent reference 2: JP-A-53-55379
Patent reference 3: JP-A-61-146522